This invention relates to a flexural transducer and, more particularly, to a flexural electromechanical transducer formed of a constant-modulus alloy material and a piezoelectric ceramic material with a large electromechanical coupling coefficient.
Mechanical frequency filters which are composed of mechanical resonant and coupling elements are now being commonly used to produce certain filter frequency characteristics and are particularly advantageous in commercial applications because of their small structural size. These filters are equipped at their inputs and outputs with electromechanical transducers so that they can be used in electric circuits. In esssence, the mechanical filters convert an electrical signal into a mechanical signal and extract an electrical output signal after filtering is achieved by the mechanical resonators and coupling elements. Ultrasonic waves is a common medium in mechanical filters. Transducers having the piezoelectric effect are also commonly used in the mechanical filters to convert the input electrical signal into the mechanical signal or to convert the mechanical signal into the output electrical signal.
A flexural transducer is particularly useful in band pass mechanical filters for low frequencies, for reasons which will be described hereinafter. Applications of a flexural transducer include use in various kinds of communication and control systems, particularly channel filters for carrier transmission and channel translating equipment. In these systems and equipment, as well as in other expanding fields of application, there is need for high performance mechanical filters having stringent filter characteristics. Such high performance mechanical channel filters, furthermore, must be highly reliable and manufactured in the most cost-effective way while ensuring the necessary filter characteristics. For example, several key factors in minimizing cost is to hold the overall filter structure to a minimum size, provide a filter which is simple in structure, standardized in dimension, and which does not require close dimensional tolerances or complicated assembly. Every component of the mechanical filter must be considered in meeting the above requirements. One key component is the transducer.
As is well known in the art, the mechanical characteristics of a piezoelectric transducer can be translated into an equivalent electrical circuit, and vice versa. An equivalent electrical circuit for a conventional transducer is depicted in FIG. 1, wherein the components L, C and R are the electrical equivalents of the mechanical properties of the transducer. More specifically, as shown in FIG. 1, L.sub.0, C.sub.0 and R.sub.0 represent respectively the equivalent inductance, equivalent capacitance, and the equivalent resistance of the series resonance of the transducer, while Cd represents the damped capacitance of the piezoelectric crystal or plate.
The performance of a transducer described above is generally measured by a capacitance ratio r and a quality factor Q, as expressed by the following:
r=Cd/C.sub.0 PA1 Q=.omega.Lo/Ro,
wherein the symbol .omega. is a series resonant angular frequency of the transducer.
In obtaining desired filter qualities for a mechanical filter using piezoelectric transducers, it is also necessary to take into consideration the realizable band width of the mechanical filter. The realizable bandwidth of a filter represented by .DELTA.f is inversely proportional to the capacitance ratio r of the transducer as expressed by the following equation: EQU .DELTA.f.varies.1/r
In view of this inverse proportion, it can be seen that a wide pass band can be achieved for a mechanical filter when the capacitance ratio r of the transducer is small.
Piezoelectric transducers now used for mechanical filters are commonly of a flexural bar-type configuration. The flexural bar-type transducers can be further classified in accordance with their structural arrangement, as will now be described with reference to FIGS. 2 and 3.
One example of a conventional flexural bar-type transducer is depicted in FIG. 2. This transducer comprises an elongated metal plate 1 formed of a constant-modulus alloy material and an elongated piezoelectric ceramic plate 2 coupled to the metal plate at one outer surface along a longitudinal direction of the metal plate. The ceramic plate 2 has a residual polarization in the thickness direction of the metal plate 1 as indicated by an arrow 3. Lead wires 6 and 7 are respectively connected to the piezoelectric ceramic plate 2 and metal plate 1. Because of the above transducer's structure, when an AC voltage of a specified frequency is applied across the lead wires 6 and 7, the piezoelectric ceramic plate 2 vibrates by "extension and compression" in a direction indicated by an arrow 4. The vibration of the piezoelectric plate in turn causes flexural vibration of the transducer shown by a dotted line 5.
The flexural transducer illustrated in FIG. 2 is principally used in narrow band pass filters for lower frequencies because it is very stable for temperature and aging variations. It is also simple in construction and easily fabricated. However, this transducer is not effective in numerous mechanical filter applications because a large realizable band width .DELTA.f and a low impedance cannot be achieved. This is due primarily to the piezoelectric ceramic plate 2 of the transducer providing electromechanical conversion of signals in the "extension and compression" vibrational mode. This phenomenon, of course, results in a very small or low electromechanical coupling coefficient of vibration, typically 0.3. The small coupling coefficient in turn results in a very large capacitance ratio r for the transducer. Since the realizable band width .DELTA.f of a mechanical filter is inversely proportional to the capacitance ratio r of the transducer, a very narrow bandwidth and high impedance results.
To widen the band width .DELTA.f of a mechanical filter employing the above-described transducer having a large capacitance ratio r and to obtain impedance matching with an external electrical circuit, an additional electrical circuit of inductance and capacitance elements, i.e., L's and C's, must be used or connected to the input/output sides of the filter. This addition, however, is unsatisfactory because the physical size of that electrical circuit usually is equal to or becomes larger than the physical size of the mechanical filter consisting of the transducers, resonators and couplers. Accordingly, the overall structure and costs of the system are increased, rather than reduced.
Another example of a conventional flexural bar-type transducer is depicted in FIG. 3. This transducer has two piezoelectric ceramic plates 10 and 11 positioned between and mechanically coupled to a pair of metal plates 8 and 9. The piezoelectric ceramic plates 10 and 11 have opposite residual polarizations, as indicated respectively by arrows 12 and 13, but are in a direction which coincides with the longitudinal direction of the transducer. Lead wires 17 and 18 are respectively connected to the metal plates 8 and 9. Because of the above transducer's structure, when an AC voltage of a specified frequency is applied across the lead wires 17 and 18, the piezoelectric ceramic plates 10 and 11 vibrate respectively as indicated by arrows 14 and 15 in a "thickness-extensional" mode. This vibration in turn causes the flexural vibration of the transducer shown by the dotted line 16.
The transducer of FIG. 3 has an electromechanical coupling coefficient of vibration, typically 0.5 to 0.6, which is larger than that of the first flexural bar-type transducer shown in FIG. 2. This is due principally to the thickness-extensional vibrational mode of the piezoelectric ceramic plate. In view of this, the capacitance ratio r of the transistor becomes smaller, and a mechanical filter having a wider pass band width .DELTA.f can be achieved without using an additional electrical circuit of inductance and capacitance elements. Despite the above advantages, the transducer of FIG. 3 has numerous disadvantages because it uses more components and requires that the pair of piezoelectric ceramic plates be assembled in the transducer with reverse residual polarization directions. Accordingly, a more complicated overall structure results, and the costs and fabrication time of the transducer increase.
As can be seen from the foregoing remarks, electromechanical transducers of high performance characteristics which are simple in construction, reliable, and easily manufactured have yet to be satisfactorily achieved for the current and expanding fields of application, such as in mechanical channel filters.